Digital imaging systems such as digital cameras utilize semiconductor chips equipped with photo-sensitive electronic components, such as photo-diodes. The digital imaging systems typically capture light information in a series of pixels. Commonly, the pixels are arranged in an array of rows and columns, such as 1024×768 pixels. Each pixel is represented by at least one photo-sensitive component. In applications requiring the capture of color, color filters may be used to capture the specific colors of the received light, and each pixel may be represented by more than one photo-sensitive component.
Generally, a microlens guides the light to the photo-sensitive component, essentially acting as the collection point for the digital imaging system. A microlens is a tiny lens formed on a semiconductor chip above each photo-sensitive component. Because the collected light passes through the microlens, it is important that the microlens be free of as many defects as possible.
FIG. 1 is a cross-section view of a semiconductor wafer configured with a microlens imaging system, although other configurations are possible. Microlenses 110 are located on a planarization layer 112, which acts as a spacer. The microlenses 110 capture light and directs the light through color filters 114 to photo-diodes 116 formed in the active region 118 of a wafer. A second planarization layer 120 may be located between the color filters 114 and the active region 118. A bond pad area 122 and scribe line 124 are typically formed as indentations in the wafer.
Microlenses are generally formed by applying a layer of microlens material on a semiconductor chip. The microlens material, such as a mixture of ethyl lactate and propyleneglycol monoether ether acetate and a mixture of propyleneglycol ether acetate and phenolic resin, also acts as a photo-resist material, i.e., the microlens material may be masked, exposed, and developed to remove unwanted microlens material. Suitable microlens material includes MFR 380 series and MFR344 series; manufactured by JSR Corporation of Tokyo, Japan.
After the microlens material has been applied, exposed, and developed, a reflow process is performed to cause the remaining microlenses to form the preferred uniform symmetrical lens shape.
In order to achieve the preferred uniform and symmetrical lens shape, it is desirable that the microlens material be applied uniformly. If the microlens material is not applied uniformly, the microlens material may not reflow evenly and may create a non-uniform lens. The non-uniform lens may cause a stripe defect, resulting in a signal deviation. In many applications, the signal deviation is too great for the devices to function satisfactorily.
The topography of the semiconductor chip has an effect on the uniformity of the microlens material. For example, if the topography of the semiconductor chip contains valleys or indentations, the indentations may cause a non-uniform distribution of the microlens material. In particular, the bond pad area and the scribe line has been known to cause an uneven distribution of microlens material.
Therefore, there is a need for a method for fabricating microlenses to reduce signal deviation caused by the semiconductor chip topography.